I would put the 4 kids big & mature enough for lap/shoulder belts along that side with those belts -- many highback boosters go up to 100 pounds & if you answer "yes" to ALL these questions then they don't need a boooster:
The 5-Step Test
1. Does the child sit all the way back against the auto seat?
2. Do the child's knees bend comfortably at the edge of the auto seat?
3. Does the belt cross the shoulder between the neck and arm?
4. Is the lap belt as low as possible, touching the thighs?
5. Can the child stay seated like this for the whole trip?
[Britax Bodyguard is a great booster offering side-impact protection*]
As for the lap-only belts with your younger children, I would think you should probably buy carseats that will hold bigger kids -- like the Britax Husky >
http://www.car-seat.org/showthread.php?s=&threadid=553
I've heard that some areas offer vouchers for carseats ... you can ask your local CPS tech about that & for more help on properly securing your children in your vehicle >
http://www.nhtsa.dot.gov/CPS/CPSFitting/Index.cfm
Also, there is the E-Z-ON Universal Harness for your kids not in carseats & without shoulder belts! >
http://www.ezonpro.com/products/familyVehicle/86yHarness.shtml
*I'm sure that with 8 kids a 15-passenger van is about the only option, but here's this if you haven't already seen >
http://www.nhtsa.gov/cars/problems/studies/15PassVans/15PassCustomerAdvisory.htm
Hope this helps & good luck